In a current reforming system for a hydrogen station, hydrocarbon is reformed to hydrogen and carbon monoxide (CO) by a water vapor reforming reaction represented by the following chemical formula 1, and further, hydrogen is produced by allowing carbon monoxide and water vapor to react using a CO shift reaction.CH4+H2OCO+3H2  Chemical formula 1
In a conventional CO shifter, as a cause that inhibits scale reduction and shortening of the starting time, need for a large amount of a CO shift catalyst due to restriction of the CO shift reaction represented by the following chemical formula 2 on the chemical equilibrium can be mentioned. As one example, in a reforming system for PAFC (phosphoric acid type fuel cell) of 50 kW, 20 L of the reforming catalyst is needed, whereas 77 L of the CO shift catalyst, which is about four times as large, is needed. This is a large factor that inhibits scale reduction of the CO shifter and shortening of the starting time. Here, the symbol “” denotes a reversible reaction.CO+H2O CO2+H2  Chemical formula 2
Thus, by providing a CO2 facilitated transport membrane that allows carbon dioxide to permeate selectively in the CO shifter and efficiently removing carbon dioxide on the right side that has been produced by the CO shift reaction of the above chemical formula 2 to the outside of the CO shifter, the chemical equilibrium can be shifted to the hydrogen production side (right side), whereby a high conversion ratio can be obtained at the same reaction temperature and, as a result of this, carbon monoxide and carbon dioxide can be removed beyond the limitation imposed by the restriction of the equilibrium.
FIG. 20 is a conceptual block diagram of a hydrogen production apparatus including a CO shifting section provided with a CO2 facilitated transport membrane. A reformer 31 receives supply of CH4 and H2O and generates a water vapor reforming reaction represented by the above chemical formula 1. A membrane reactor 30 receives supply of a mixture gas containing H2 and CO2 that have been produced in the water vapor reformer 31 and residual H2O, and generates a shift reaction represented by the above chemical formula 2 in a shift treatment section 32. Here, the membrane reactor 30 is provided with a CO2 facilitated transport membrane 33 that allows CO2 to permeate selectively. By this, CO2 produced by the chemical formula 2 permeates through the membrane 33 to be discharged to the outside together with an inert sweep gas (for example, Ar gas). Also, by this, by recovering a gas that has not permeated through the membrane 33 from the shift treatment section 32, H2 gas having a small content of CO2 and a small content of CO can be obtained.
FIG. 21 shows concentration change of each of carbon monoxide (A) and carbon dioxide (B) along the catalyst layer length of the CO shifter when provided with the CO2 facilitated transport membrane and when not provided with the CO2 facilitated transport membrane.
By the CO shifter provided with the CO2 facilitated transport membrane (CO2 permeation type membrane reactor), carbon monoxide and carbon dioxide can be removed beyond the limitation imposed by the restriction of the equilibrium. This can achieve reduction of the load of PSA and S/C in the hydrogen station, so that the cost reduction and higher efficiency of the whole hydrogen station can be achieved. Also, by incorporating a CO2 facilitated transport membrane in a shifter, increase in the rate of the CO shift reaction (higher SV) can be achieved, so that the scale reduction of the reforming system and the shortening of the starting time can be achieved. For example, as a prior example of such a CO2 permeation type membrane reactor, there is one disclosed in the following Patent Document 1 (or Patent Document 2 with the same contents by the same inventor).